Set up an ongoing Firefox 57 (Quantum, desktop) set of test runs

To be able to compare to Quantum's performance in Firefox 57 to current release + Nightly, we should add a Firefox 57 (starting with desktop) run, alongside our current ones.

We (WebPagetest, wptagent, and webpagetest-api) don't yet have browser versioning set up, but I know Browsertime/sitespeed.io has latest/latest-n versioning set up via Docker:

https://www.sitespeed.io/documentation/sitespeed.io/browsers/

Docker "base" image for the browsers: https://hub.docker.com/r/sitespeedio/webbrowsers/dockerfile

Dockerfile for above Docker image/container: https://github.com/sitespeedio/docker-browsers/blob/d250e26747f1b72e08d1ddb07694f2ab603624a8/Dockerfile#L7